It seems that even some long time users have not had any idea how to use the dirt maps. This tutorial will attempt to shed some light on this subject. So first we need to know what defines a dirt map. The basic characteristic of all the dirt map presets is that they are monochrome images. This presents many possible ways to use the dirt maps for enhancing our images. The thing to remember is that a dirt map is rarely used alone.

All the textures in this tutorial will have the key concept at the top level and any input texture will be a group with a name to explain what it is. By studying the gtx files in combination with this text you should be able to see what is going one.

Let’s start with the “foot prints” dirt map preset. This can be used in many ways but one of the most interesting ways is to use it as a height map. By using this as the input to a point light node you can make the foot prints become indentations. By following this with colorize and combine nodes you are set to put the indentations onto the texture of your choice. In the example here I have used the sandy beach texture as the input. I set the combine node to mix using a multiply then follow the combine node with a change bci node to bring the brightness of the image back up. I now have footprints on the beach.

Using a combine node with one of the inputs being a dirt map is the second way to use them. By combining the dirt maps with a texture using multiply or least, etc combine modes you can get several different effects. The next example is oil on a metal plate. The combine mode is set to multiply for this to show dark oil on the metal. This can be used for any of the dirt maps. It basically creates a dark overlay to the base image.

The third way to use dirt maps is probably the most rewarding. In this method you use the dirt map as the selector input on a combine. This allows you to mix textures using the dirt mask as a mask. For this example I will use the oily film again but this time I will use it to select a oily film transparent overlay to add to the metal plate. While I could have used it directly to select between two materials like with say goo or grungy boarder the oily film lends itself to this technique but I will make an example that uses the grungy boarder dirt map to select between to materials.

So for the oily film the first thing I do is to use the dirt map to create an image that is transparent where there is no dirt and white where there is dirt. The next trick is to combine this with the oil color map using a combine node set for multiply and opacity dropped some. This creates a colored film then this is simply overlaid on a metal material.

On the two material select it is just a combine node with to different materials as input and the dirt map as the selector. This is very simple but it works very well for things like goo, secondary splatter, etc.

Thank you for the great tutorial! New users are likely to not realize where dirt maps are found, so I inserted an illustration at the top of your tutorial that shows where they are found--I hope you don't mind.

The way the presets create them they are monochrome images (black & white) They can be used to create an alpha channel by using a combine node but the dirt maps are not intrinsically an alpha channel.

That's right. It's also worth mentioning that artists sometimes like to keep dirt maps as a separate texture to layer on top of the base in the actual 3D app or game engine so that the dirt layer can be given a different size and orientation, thus helping break up the appearance of repetitions in the base texture. In this example you can see that the underlying metal texture is repeated 9 times, but that fact won't pop out at you as easily as it would if the rotated dirt map, which is itself repeated multiple times, weren't layered on top of it.

Here another example of how the dirt map can be used as a separate texture. In this case the dirt map is placed only once as a unique blemish to help hide the fact that the wood texture is repeated 9 times.